Magnetic Field Fluctuations Measurements in the Solar Orbiter Project

Size: px

Start display at page:

Download "Magnetic Field Fluctuations Measurements in the Solar Orbiter Project"

Mitchell Oliver
5 years ago
Views:

1 Magnetic Field Fluctuations Measurements in the Solar Orbiter Project 1 Pinçon JL, 1 Dudok de Wit T, 1 Krasnoselskikh V, 1 Seran HC, 1 Cavoit C, 1 Fergeau P, 1 Chabassiere M, 2 Roux A, 2 Berthomier M,, 2 Chust T, 2 Le Contel O, 2 Rezeau L, 2 Coillot C, 3 S. Bale 1 LPCE/CNRS, 3A Av. de la Recherche Scientifique, Orléans, France 2 CETP/IPSL/CNRS, 10/12 Avenue de l Europe, Vélizy, France 3 SSL, Berkeley, USA Abstract. Although much is known concerning the waves that exist in the solar wind near the orbit of the Earth, many questions remain concerning the waves occurring much closer to the Sun. At the present time the measurements closest to the Sun have been obtained from the Helios 1 and 2 spacecraft at approximately 0.3 AU. Based on these observations, hereafter is a presentation of the characteristics of the waves expected to be present at 0.2 AU and the magnetic field sensors required to measure them. VLF magnetic sensor for Solar Orbiter ELF/VLF waves & turbulences: Alfvén turbulence and ion-cyclotron waves play an essential role in the heating and acceleration of the solar wind. Recent studies of the particle distributions measured onboard Helios have evidenced the process of pitch angle diffusion of protons (Tu and Marsch 2001). They give strong argument that the plasma heating caused by Alfvén turbulence and Ion- Cyclotron Waves activity continues to be effective on quite large distances from the Sun. These waves are generated below and at the local proton gyrofrequency, respectively. At 0.2 AU the value of ion gyro-frequency Ωci can be estimated to be about 1 Hz (Gurnett 1978). Actually, the

2 corresponding frequencies observed in the spacecraft frame can be Doppler shifted up to the khz range. The maximum amplitude for Alfvén and ion-cyclotron waves should be of the order of 10-2 nt/(hz) 1/2. Whistler turbulence and Ion-Acoustic waves: Above the proton gyro-frequency Ωcp, the next resonant frequency to be encountered is the electron gyro-frequency (Ωce = 2 khz at 0.2 AU). For frequencies between Ωcp and Ωce the only electromagnetic mode that can propagate is the right-hand polarized whistler mode. Measurements from Helios have shown that whistler turbulence is present in the solar wind at frequencies up to Ωce. Helios observations stressed the trend for most solar wind wave modes to increase in intensity at smaller heliocentric distances. Based on these trends one can estimate the whistler mode emissions at 0.2 AU to extend from 10-5 nt/(hz) 1/2 up to 10-1 nt/(hz) 1/2 (see Gurnett, Plasma waves near the Sun, 1978). Measurements from Helios also revealed the occurrence of ion-acoustic-like electrostatic waves in the solar wind at frequencies between the electron and ion plasma frequencies (at 0.2 AU, fpi = 4 khz and fpe = 200 khz). Although the ion-acoustic mode propagates at frequencies below fpi, the observed wave frequencies are mainly determined by the Doppler shift, which is much larger than fpi. As pointed out by Kellog et al. (1999), magnetic field measurements will be necessary to unambiguously determine whether the observed waves are electromagnetic or electrostatic. The estimate of the amplitude of the magnetic field component in these waves shows that it is large enough (about 10-4 nt/(hz) 1/2 ) and can be easily measured by the 3-axis ELF-VLF magnetic sensors we propose below. The choice of the instrument characteristics will be determined by the combination of factors such as the accommodation place available for the instrument, electromagnetic compatibility with the plasma environment, radiation and thermal requirements and the noise level around it. Figure 1 shows a recent development of a three component magnetic sensor which has its preamplifier placed inside the support (Seran and Fergeau 2005). Each magnetic antenna is 17 cm long. The miniaturized 3 channel preamplifier itself is shown in Figure 2. The main characteristics of the ELF- VLF search coil are presented in the Table 1. Figures 3 and 4 represent comparative frequency responses and noise levels of the sensors with lengths of 10 cm and 17 cm. Onboard data processing can be performed by special dedicated unit that can be realized using 3D+ technology. More details can be found in the Thesis work of Vassal (2001).

The orange (top) and blue (bottom) lines correspond to antennae that

3 Fig. 1. The 3-axis ELV-VLF search coil. Each coil is 17 cm long Fig. 2. The miniaturized 3 channel preamplifier Fig. 3. Frequency response of the ELF-VLF search coil. The orange (top) and blue (bottom) lines correspond to antennae that are respectively 17 cm and 10 cm long

4 Fig. 4. Noise level of the ELF-VLF search coil. The orange and blue lines correspond to antennae that are respectively 17 cm and 10 cm long HF magnetic sensor for Solar Orbiter Another important scientific objective is related with the problem of particle acceleration, in particular, in association with Coronal Mass Ejections (CME s), during and after flares. Different acceleration mechanisms acceleration can occur, such as acceleration by collisionless shocks, or acceleration associated with the CME propagation such as by a potential electric field along the magnetic field lines. Accelerated electrons generate high frequency electrostatic and electromagnetic waves. The Solar Orbiter mission gives the possibility to carry out a systematic study of the wave activity in the frequency range (below 20 MHz) that cannot penetrate through the ionosphere. Such waves are registered onboard the Wind satellite, whose observations give excellent examples of radio emissions associated with flares and CME s that propagate in the interplanetary medium. These observations are based on one component of the electric field, with a 15 m tip-to-tip antenna, and allow to track the propagation of electron beams injected into the interplanetary medium after a flare has occurred. Such observations, combined with ground based observations by radio-telescopes, allow one to follow the evolution of the shocks using type II bursts. They give the opportunity to carry out complementary studies of the characteristics of electron acceleration processes, and the characteristics of the beam propagation. Figure 5 represents combined measurements of a type III and

5 Fig. 5. Combined measurements of type III / type II bursts observed onboard Wind satellite (RAD 1 and RAD 2 instruments) and by IZMIRAN radio-telescope A type II burst observed onboard Wind satellite by RAD 1 and RAD 2 instruments and by the IZMIRAN radio-telescope. The characteristic intensity of these signals can be estimated, taking into account that the maximum of the cosmic background spectral intensity is about W/ (m 2 Hz 1/2 ) at 3 MHz, which corresponds to an electric field spectral density equal to ( ) 2 V 2 /m 2 Hz (Dulk et al. 2001; Manning and Dulk 2001). The typical intensity of the observed signals is 5 10 db above this value, which implies an intensity of the order of nv/(m Hz 1/2 ). For a refractive index equal to one, the characteristic amplitude of the corresponding magnetic field can be estimated to be about nt/hz 1/2. The Wind measurements were performed around 1 AU. Taking into account that Helios observations show that the wave intensities associated with type III radio-bursts increase very rapidly with decreasing radial distance from the Sun, one can evaluate the amplitude range of the expected wave signals. In the frame of the Solar Orbiter project, intense HF emissions should reach amplitudes up to 10-4 nt/(hz) 1/2. We present here a very sensitive magnetic loop antenna developed by LPCE to measure the magnetic field of these high frequency waves. Several magnetic sensors operating in the frequency range up to several MHz have been used in space projects. A rectangular magnetic loop antenna was used in Polar project by Iowa University group (Gurnett et al. 1995). Our circular coil antenna realized in LPCE has a diameter of 20cm (Figure

6) and a sensitivity equal to 0.3 10-6 nt/hz 1/2. Figures 7 and 8 respectively show its frequency response and noise level. The antenna sensitivity can be improved by increasing its size.

6 6) and a sensitivity equal to nt/hz 1/2. Figures 7 and 8 respectively show its frequency response and noise level. The antenna sensitivity can be improved by increasing its size. A simulation of the characteristics of an antenna having a square shape and 50cm size is presented on Figure 8; it gives a quite optimistic evaluation of the possibility to measure electromagnetic waves in the type II an type III bursts, in a wide frequency range extending from 100 khz up to 30 MHz. The sensitivity of magnetic loop antennas is significantly less than that of the electric antennas. The high level of galactic noise however, makes this difference less important, because it does not allow the whole dynamic range of electric antennas to be used in such measurements. Magnetic antennas also have several advantages: accommodation of such an instrument is quite simple: its shape can be adapted to the surface of the non conducting shielding and it can be placed close to the surface without use of the special boom. Another important factor is related with the noise generated around the satellite by the evaporation of the shielding. This noise is mostly electrostatic and so is more likely to affect electric field measurements than magnetic field measurements. The characteristics of the instrument are summarized in the Table 1. Fig. 6. HF magnetic loop with preamplifier. The loop diameter is 20 cm

7 Fig. 7. Frequency response of the HF magnetic loop. The full and dashed lines respectively correspond to the modulus and the phase of the transfer function Fig. 8. Noise level of the HF magnetic loop. The full line corresponds to the measured values, for a circular loop with a diameter of 20 cm. The dashed line corresponds to the simulated values, for a square shaped magnetic antenna of 50 cm side Conclusion. The prime scientific objectives of Solar Orbiter project include the experimental study of the heating of the coronal and solar wind plasmas due to wave activity, and electron acceleration. To solve these problems it is ne-

8 cessary to measure particle distributions and electromagnetic fields in a wide frequency range from several Hertz up to MHz. We propose two instruments to measure the magnetic field of electromagnetic waves in this frequency band. They are a three component search coil in the range 3 Hz 20 khz and a very sensitive one component magnetic loop covering the frequency band from 10kHz up to 20MHz. We show that using this instrumental set we can meet the requirements determined by the scientific objectives of the mission. The authors acknowledge useful and helpful comments of Kaiser and Thejappa concerning RAD 1 and 2 experiments onboard Wind. Table 1. Characteristics of the ELF-VLF search coil and the HF magnetic loop 3axis ELF-VLF search coil HF magnetic loop Bandwidth Sensitivity 1Hz 20kHz 10-5 nt/hz 1/2 at 1kHz 10kHz 20MHz 10-6 nt/hz 1/2 at 1MHz Dimension L=18cm =2cm =20cm Mass sensors=3 100g preamp=20g Total=200g Power 100mW (±12V) 300mW (±5V) References. Dulk GA, Erickson WC, Manning R, Bougeret JL (2001) Calibration of low-frequency radio telescopes using the galactic background, Astron. & Astrophys., 365, p 294 Gurnett DA (1978) Plasma waves near the Sun. In: Neugebauer M and Davies RW, (eds) A close up to the Sun, JPL Publ., pp Gurnett DA (1991) Physics of the Inner Heliosphere. Springer-Verlag, Berlin, pp. 135 Gurnett DA, Persoon AM, Randall RF, Odem DL, Remington SL, Averkamp TF, Debower MM, Hospodarsky GB, Huff RL, Kirchner DL, Mitchell MA, Pham BT, Phillips JR, Schintler WJ, Sheyko P, Tomash DR (1995) The Polar plasma wave instrument, Space Sci. Rev.,71: Kellog PJ, Goetz K, Monson SJ, Bale SD (1999), Langmuir waves in a fluctuating solar wind, J. Geophys. Res., 104: Manning R, Dulk GA (2001) The galactic background from 0.2 to 13.8MHz, Astron. & Astrophys., 372, p 663 Seran HC, Fergeau P (2005) An optimized low frequency three axis search coil magnetometer for space research, accepted for publication in Review of Scientific Instruments Tu CY, Marsch E (2001) Wave dissipation by ion cyclotron resonance in the solar corona, A. & A., 368: Vassal MC (2001) Etude et miniaturisation d un analyseur basse fréquence destiné à la correction de l effet Doppler à bord d une sonde en mouvement rapide par rapport au milieu étudié, Thèse de l Université Versailles Saint-Quentinen-Yvelines

SURA-WAVES experiments: calibration of the Cassini/RPWS/HFR instrumentation

SURA-WAVES experiments: calibration of the Cassini/RPWS/HFR instrumentation Abstract The SURA facility transmitted signals in the 9 MHz frequency range to the Cassini spacecraft during the Earth flyby